Building-Mounted Wind Turbines: Structural Challenges

The mass media seem to love stories about building-mounted wind turbines, even though bona fide manufacturers, professional installers and knowl­edgeable rebate program managers actively discourage their purchase and installation. While inventors of such “breakthrough technologies” claim that the mainstay small wind industry is just trying to squash potential com­petition, the real reasons are actually quite different.

Rooftop wind turbines will never be a viable technol­ogy for several reasons. First, the small collector size of these devices (a requirement so that the weight of the turbine does not col­lapse the roof) means that only small amounts of fuel — the wind — can be accessed and converted into electricity. Second, wind strength is inadequate at rooftop height to produce significant power. Rooftop wind is also turbulent, and therefore quite destructive of wind turbines. This combination of limita­tions means that rooftop turbines can never be cost-effective. Any number of small wind test sites have confirmed these points.

Now a new problem is emerging: Permitting authorities are receiving applications from homeowners and businesses that want to greenwash their images with building-mounted turbines. Authorities are therefore wrestling with how to write appropriate ordinances.

Zoning and permitting language for standard tower-mounted wind turbines is quite straightforward. The common standard is that the setback for the tower should match the total structure height from property lines unless an adjoining property owner agrees to a shorter distance. What do you do, however, with a wind turbine that is advertised to be mounted on top of a roof in an urban area where property lots are 25 feet to 50 feet (7.6 meters to 15 meters) wide?

There’s a more serious problem: Small wind sys­tems from mainstream manufacturers are engineered structures, designed with a built-in safety factor. The engineering designs cover all components of the wind systems including the turbine, electronics (inverters and controllers), tower and foundation. Tower and foundation are engineered to the specifications of the International Building Code (IBC), which dictates the extreme winds, ice loads, peak gusts and allowable load­ing on the structure.

What does a permitting authority do with a turbine that may (or may not) be engineered, but is mounted on a structure that was clearly not designed to support a wind turbine, nor the loads developed by that turbine when it is spinning and generating electricity? This is a legitimate concern, as there have been reports of building-mounted turbines seriously damaging the roof or wall they were mounted on, in some cases endanger­ing nearby properties.

Residential roofs and commercial building structures were designed for certain loads specific to the building and location, including —

• Dead loads: The weight of the structure plus the addi­tional roof layers and HVAC equipment added over time.

• Live loads: Wind, traffic movement and earth tremors, and occupational loads including workers on the roof.

• Environmental loads: Snow and ice accumulation, temperature changes and rainfall. Since these loads are dif­ficult to model, the IBC usually specifies 50-year extremes.

• Dynamic loads: Severe storms, impacts, wave action near large bodies of water and earthquakes. These are extremely complex loads to model as they are unpredict­able as well as varying in frequency, amplitude, duration — and may or may not have a cyclic nature. The IBC specifies considerations for some of these loads, but usu­ally defers to a specific engineering analysis to account for them. The prudent building owner purchases insurance for such extreme occurrences. A wind turbine on a roof or building would be categorized as a dynamic load not accounted for in the IBC or building structural design.

So how does a permitting authority deal with this?

Unlike engineered tower-mounted wind systems, the structure that a rooftop wind turbine is to be mounted on needs to undergo a complete engineering analysis for the specific wind turbine. This must include the loads placed on the roof, walls, foundation and associated struc­tural support members by the wind turbine; all shear loads and overturn moments; the centrifugal forces and resonant frequency of the spinning rotor and governing mechanisms; the yaw and generator torque developed by the turbine; and the aerodynamic thrust loads generated by the wind system. Only such an analysis will guarantee the same level of safety engineered into tower-mounted wind systems.

This article appeared in the March/April 2012 issue of SOLAR TODAY. Subscribe today!